This invention relates to radio ranging electronics equipment in general, and more particularly to pulsed Doppler radar systems.
Radars were developed to detect and determine the range of aircraft but in recent years their beams have become more directive, the receivers more sensitive, and transmitters have become more coherent. Radars have been successfully utilized in mapping storm structures showing the respective hydrometeors which can be correlated to map incipient storm activity. As these radars become more widely used, the improved coherency of the transmitters results in a severe alien radar noise problem which hithertofore has been successfully approached using noise suppression techniques.
Alien reception by a radar receiver tuned to the same frequency as another radar systems' transmitter is an often-encountered condition which is understood by all in the industry. Generally the solution to this type of interference has been one of tolerance as opposed to positive design measures, other than noise considerations and filtering, tailored to eliminate the problem. As transmit/receive frequencies have become more stable, and coherent systems more prevalent, the stability of the systems and the greater number of systems of the same type in service have combined to make the alien radar situation a severe problem. The frequency stability, same pulse repetition frequency, longer pulse widths, and memory supported displays, combined to assure reception of all similar type radars in the vicinity of the user with detrimental effects on the display. In the worse case situation, the display of the desired data becomes so contaminated with alien inputs as to become essentially useless.
The development of high coherency transmission capabilities has resulted in multiple stations having substantially identical equipments transmitting on very close, or the same, frequencies. Since the radar beams are highly directive, a significant amount of radio energy is concentrated in the receiver of a first radar when an alien radar is transmitting in the vicinity. This results in a broad smear indication on an uncorrected radar receiver screen and noise techniques are generally inadequate to eliminate such high energy multiple range bin alien signals. A comprehensive basis for the development and application of radar techniques is found in the text entitled, Introduction to Radar Systems by Merrill I. Skolnik, 2nd ed.,1980.
The application of various PRFs and pulse duration as a function of desired range and resolution information is well known in the art to obtain a higher PRF and narrower pulse width for high resolution on close targets and a lower PRF and wider pulse width on long-range targets assuming the target depths compatible with the pulse width selected. The PRF is a function of the time required for the signal to travel the path to the target and back. PRF decreases as range increases, as time must be allowed for the range return. Pulse width also may increase in range to obtain more range capability. Accordingly, those variable PRF systems are controlled as a strict function of the desired range indication, or possibly as a function of resolution desired. The alien radar problem can become severe in situations in which two radar equipments are substantially the same, as for example when manufactured by the same company and further are operating in relative close proximity.